To convert hydrocarbons into petrochemical or basic chemicals, chemical conversion processes are typically utilized. These processes involve using thermal or chemical reactors to produce products, such as acetylene, ethylene and/or propylene in different proportions. As an example, pyrolysis is a conversion process in which at least a portion of the hydrocarbons in a stream is converted (e.g., cracked) at an elevated temperature into unsaturated hydrocarbon molecules, such as C2 unsaturates. Pyrolysis typically occurs at operating temperatures above 430° C.
Different types of processes may be utilized to crack hydrocarbons. These processes can be divided into four different types: partial combustion processes, indirect combustion processes, arc processes, and thermal pyrolysis processes. These pyrolysis process types differ in the means utilized for generating and transferring the heat for the pyrolysis and/or in the severity of the operating conditions (e.g., low-severity and high-severity).
While various different types of processes may be utilized, certain processes may be preferred based on the feed. For instance, because certain hydrocarbon containing feeds are more expensive than others, it is desirable to use lower cost, heavier hydrocarbon containing feeds in pyrolysis processes to produce unsaturate products having two carbon atoms (C2). The heavier hydrocarbon feeds tend to contain higher amounts of non-volatiles (e.g., combustible non-volatiles and non-combustible non-volatile) and tend to form higher amounts of coke, ash and tar, which make it more difficult to pyrolyze the heavier hydrocarbons containing feeds due to the formation of the coke, ash and tar deposits in the pyrolysis reactors.
The partial combustion processes, indirect combustion processes and arc processes are able to handle wider varieties of hydrocarbon feeds, such as heavier hydrocarbon feeds that contain non-volatiles, because these processes do not rely upon heat transfer through a solid material to transfer sufficient heat to crack the hydrocarbon. Rather, these types of processes involve the mixing of combustion products and pyrolyzed hydrocarbon products together (e.g., utilize pyrolysis and combustion chemistry) or utilizing plasma arcs or electric arcs to supply sufficient heat to crack at least a portion of the hydrocarbons in the feed to unsaturated compounds. Examples of these processes include U.S. Pat. Nos. 1,860,624; 3,242,223; 3,419,632; 7,119,240; and 7,208,647. Accordingly, these processes typically involve equipment that provide larger flow passages, which assist in managing the non-volatiles within the streams in the reactor. Limitations of these types of processes, however, include one or more of higher operating expenses, poor energy efficiency and additional separation equipment required to remove undesired byproducts (e.g., combustion products) from the pyrolyzed hydrocarbons.
Thermal pyrolysis processes provide enhancements to other pyrolysis processes by maintaining the combustion products and pyrolyzed hydrocarbon products separate through the process. Examples of thermal pyrolysis processes include U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; 3,024,094; 3,093,697; 7,138,047; and 7,119,240 and U.S. Patent App. Pub. No. 2007/0191664. As these thermal pyrolysis processes rely upon the transfer of heat via a heated solid material to crack the hydrocarbons within the feed (e.g., via pyrolysis chemistry alone or primarily pyrolysis chemistry), the exchange of heat via the surface area of the solid material limits the size of the flow passages for the streams. That is, thermal pyrolysis reactors are well suited for volatized or volatizable feeds that are substantially free of non-volatile compounds because non-volatiles may cause fouling. As an example, upstream of the pyrolysis, the non-volatiles (e.g., ash and asphaltenese) in the feed may lay down along the flow path at the dry point, while during or after pyrolysis, the non-volatiles may form carbon residue or coke or lay down within the flow passages along the flow path. Further, any additional ash may also lay down along the flow path as the hydrocarbons are pyrolyzed. Because of the limited flow area in the flow passages for this process, any non-volatiles within the feed may result in fouling problems and inhibit operation of the system.
To handle the feeds that contain non-volatiles in thermal pyrolysis processes, various different processes have been utilized. For instance, steam cracking processes utilize separation techniques to remove the non-volatiles upstream of the radiant section of the steam cracking furnace because the steam cracking process does not effectively handle non-volatiles. As an example, U.S. Pat. No. 7,138,047 describes a process for cracking heavy hydrocarbon feed, which mixes heavy hydrocarbon feed with a fluid, such as a lighter hydrocarbon or water, to form a mixture stream. The mixture stream is flashed to form a vapor fraction and a liquid fraction, with the vapor fraction being subsequently cracked to provide olefins.
In addition to the separation of non-volatiles, other thermal pyrolysis processes utilize steam to manage the coke formation. As an example, U.S. Pat. No. 7,648,626 describes a process to reduce fouling associated with a steam cracking process using a specified amount of steam. The process involves establishing a ratio of total dilution H2O to feed, injecting a first portion of the total dilution H2O into the convection section of the cracking furnace, and then injecting a second portion of the dilution H2O into the convection section of the furnace. The amount of dilution H2O in the form of a ratio of liquid H2O to steam is adjusted to maintain a desired temperature profile across the convection section of the furnace to lessen problems due to fouling in the furnace.
In a thermal pyrolysis process, which utilizes a regenerative reactor, the process may also include the removal of non-volatiles upstream of the regenerative reactor. As an example, U.S. Pat. No. 7,914,667 describes a thermal pyrolysis process that reduces fouling problems in a regenerative reactor. The process describes heating a non-volatile-containing feed upstream of the regenerative reactor to a temperature sufficient to form a vapor phase that is essentially free of non-volatiles and a liquid phase containing the non-volatiles; separating the vapor phase from the liquid phase; and feeding the separated vapor phase to the reactor for conversion. The disadvantage of this process is that (i) it removes 5 to 10 weight percent (wt %) of the feed as a bottoms stream to facilitate separation and (ii) the resulting viscous bottoms stream requires fluxant to be disposed as downgraded fuel oil, coker feed or other low value stream. Accordingly, this process may unnecessarily waste feed depending on the amount of non-volatiles in the feed (e.g., less than or equal to 5 wt %).
It is therefore desirable to provide enhanced processes capable of producing higher levels of unsaturated hydrocarbon compounds using heavier hydrocarbon containing feeds, which may include a liquid portion and/or non-volatiles, without having to produce a lower value bottoms stream. Also, as thermal pyrolysis regenerative processes can be operated at higher temperatures relative to conventional steam cracking processes to produce higher levels of unsaturated compounds, it is also desirable to utilize these processes to further enhance feeds that may be utilized in these systems. Further, a regenerative process may also be utilized to enhance the management of the coke and tar formed by the pyrolysis of hydrocarbons in the feed.